Lightwave communication systems are presently designed with a requirement for lasers which have the desirable characteristics of limited wavelength tunability and single longitudinal mode operation. For a laser to be considered operating nominally in a single longitudinal mode, it is understood that the side longitudinal modes show a degree of suppression to a substantially insignificant power level relative to the main longitudinal mode emitted by the laser. The ratio of main longitudinal mode power to side longitudinal mode power is termed side mode suppression ratio or, less frequently, sub-mode suppression ratio and and is abbreviated SMSR in most technical articles.
At the present time, it is not unusual for distributed Bragg reflector (DBR) lasers and distributed feedback (DFB) lasers to have a side mode suppression ratio specified as exceeding 30 dB. However, it is important to note that the side mode suppression ratio is specified for laser operation at a particular wavelength. When the laser is tuned to a different operating wavelength, it is shown that the side mode suppression ratio changes. See, for example, IEEE J. of Quant. Electron., Vol. QE-24, No. 12, pp. 2423-32 (1988). Single mode lasers such as semiconductor DFB and DBR lasers, for example, exhibit acceptable side mode suppression at certain wavelengths away from the particularly specified wavelength but unacceptable side mode suppression when tuned to other wavelengths away from the particularly specified wavelength. One reason that the side mode suppression is compromised at the latter wavelengths during tuning is that the relative net threshold gain required for each mode varies with wavelength as the laser is tuned. When the main longitudinal mode of the laser is centered within the reflection characteristic for the resonant cavity of the laser, side mode suppression is usually optimized at a maximum value. For DBR lasers, this centering corresponds to positioning the photoluminescence wavelength at the Bragg wavelength. As the laser is tuned away from the optimum position for maximum side mode suppression, one of the side longitudinal modes is moved closer to a center position of the reflection characteristic. Since the reflection characteristics and the related net threshold gain characteristics are substantially symmetric about a central position, a condition may arise during tuning wherein both longitudinal modes near the central position in the reflectivity characteristic experience substantially the same degree of reflectivity. That is, both modes are situated at substantially equal reflectivities which occur on opposite sides of the symmetric reflection characteristic. When this condition occurs, both modes require substantially equal amounts of net threshold gain. As a result, side mode suppression is considerably degraded and the side mode suppression ratio is rapidly reduced. In our simulations with DBR lasers, for example, a small tuning change has been shown to cause a 10 dB to 20 dB drop in the side mode suppression ratio.
For production semiconductor lasers, severe fluctuations of the side mode suppression ratio during tuning pose a significant problem. A batch of lasers fabricated on the same wafer are known to have different operational characteristics. Usually such related lasers require tuning to some degree to cause them to emit light at the same wavelength. From the problems associated with tuning described above, it is expected that side mode suppression for lasers in the same batch will be different and degraded when all the lasers are tuned to operate at the same wavelength.